The present invention relates to a signal processing circuit built-in light-receiving element.
Circuit built-in light-receiving elements each incorporating a signal processing circuit and a light-receiving element in a single chip have been widely used for a photodetector, a photocoupler, and the like.
FIG. 5 shows the basic arrangement of an optical disk apparatus using a circuit built-in light-receiving element. Referring to FIG. 5, in an optical pickup 51, a laser beam emitted from a semiconductor laser 52 is focused on an optical disk 70 through a grating plate 53, a half prism 54, a collimator lens 55, and an objective lens 56. The reflected beam from the optical disk 70 passes through the objective lens 56 and the collimator lens 55, and is reflected by the half prism 54 to be incident on a photodetector 58 constituted by the circuit built-in light-receiving element through a cylindrical lens 57.
As shown in FIG. 6, the photodetector 58 comprises four photodiodes 9a to 9d arranged in a 2.times.2 array, and two diodes 9e and 9f arranged on the two sides of the photodiodes 9a to 9d in order to perform focus servo control of driving the objective lens 56 in a direction vertical to the disk, and tracking servo control of driving the objective lens in the radial direction of the disk. This optical apparatus is disclosed in, e.g., Japanese Patent Laid-Open No. 7-93771.
FIG. 7 shows the circuit configuration of the photodetector 58. In FIG. 7, the photodiodes 9a to 9f are respectively connected to the inverting terminals of current-voltage conversion amplifiers 10a to 10f, and also connected to output terminals Voa to Vof via feedback resistors 11a to 11f. A reference voltage Vc is applied from a reference power supply (not shown) to the non-inverting terminals of the amplifiers 10a to 10f. Since the amplifiers 10a to 10f have large gains, the potentials of the inverting terminals (cathode potentials of the photodiodes 9a to 9f) are also equal to Vc. The voltage Vc is generally 1/2 a power supply voltage Vcc.
Assume that the resistance values of the resistors 11a to 11f are represented by R. When photocurrents I generated in the photodiodes 9a to 9f flow through the amplifiers 10a to 10f, an output voltage I.times.R+Vc is generated at the output terminals Voa to Vof of the amplifiers 10a to 10f. The frequency characteristics of the output voltage generated when an AC optical signal is input are determined by the characteristics of the amplifiers 10a to 10f.
FIG. 8 shows a conventional circuit built-in light-receiving element constituting the photodetector 58. In FIG. 8, a photodiode serving as a light-receiving portion, and an npn transistor constituting a signal processing circuit portion (the current-voltage conversion amplifiers 10a to 10f in FIG. 7) are formed on a p.sup.- -type substrate 21. Note that the photodiodes and the current-voltage conversion amplifiers are connected as shown in FIG. 7.
A method of manufacturing the circuit built-in light-receiving element having this arrangement will be described below. An n.sup.+ -type buried diffusion layer 22 is formed in a prospective npn transistor formation region on the p.sup.- -type substrate 21. An n.sup.- -type epitaxial layer 24 is formed on the entire surface of the p.sup.- -type substrate 21 including the n.sup.+ -type buried diffusion layer 22.
A p.sup.+ -type diffusion layer 25 for separating respective elements is formed at the boundaries between the respective elements so as to extend through the epitaxial layer 24. An n.sup.+ -type diffusion layer 26 for the collector of the npn transistor is formed to reach the n.sup.+ -type buried diffusion layer 22 through the n.sup.- -type epitaxial layer 24. In the region of the signal processing circuit portion, a p.sup.+ -type diffusion layer 27 serving as the base of the transistor is formed in the n.sup.- -type epitaxial layer 24. An n.sup.+ -type diffusion layer 28b serving as the emitter is formed in part of the p.sup.+ -type diffusion layer 27. At the same time, an n.sup.+ -type diffusion layer 28a for the cathode of the photodiode is formed in part of the region of a light-receiving portion on the n.sup.- -type epitaxial layer 24.
The reference potential Vc is applied to the n.sup.- -type epitaxial layer 24 of the light-receiving portion through the n.sup.+ -type diffusion layer 28a, while a GND potential is applied to the p.sup.- -type substrate 21. In this manner, a photodiode having the p.sup.- -type substrate 21 as the anode, and the n.sup.- -type epitaxial layer 24 as the cathode is formed.
At the light-receiving portion, a beam incident downward on the chip reaches the p.sup.- -type substrate 21 through the n.sup.- -type epitaxial layer 24. Light generation carriers generated in the depletion layer of this photodiode are accelerated by the electric field inside the depletion layer, and contribute to a high-speed photocurrent. However, carriers generated deep (outside the depletion layer) in the p.sup.- -type substrate 21 gradually spread by diffusion, and some reach an upper photodiode, while others reach an adjacent photodiode.
This diffusion current decreases the light reception response time, and also becomes a crosstalk current. At present, the thickness of the n.sup.- -type epitaxial layer 24 by a general circuit process is about 3 .mu.m. Since the concentration of the p.sup.- -type substrate 21 is about 10.sup.15 cm.sup.-3, the depletion layer widens to only about 3 to 4 .mu.m. The absorption length of a 780-nm wavelength beam serving as a CD (Compact Disk) light source to Si is 10 .mu.m. Therefore, it is found that a large amount of diffusion current is undesirably generated.
FIG. 9 shows the frequency characteristics of the circuit built-in light-receiving element in FIG. 8. The frequency characteristics show the gain of the output voltage (output from the current-voltage conversion amplifier) for the frequency of an input beam. It is found that the response of the photodiode delays due to the influence of the above-described diffusion current even in a range wherein the frequency is equal to or lower than the cutoff frequency of the signal processing circuit (current-voltage conversion amplifier), and that the gain gradually decreases.
In recent years, as the rotation speed and density of an optical disk increase for an increase in data transfer rate, a higher frequency band is required for a photodetector. In the characteristics shown in FIG. 9, the gain changes depending on the signal frequency. When such a photodetector is used in the optical pickup of the optical disk apparatus, the response changes depending on a high or low speed of a reflected beam from the disk, resulting in poor jitter characteristics.
FIG. 10 shows the photosensitivity map of the circuit built-in light-receiving element in FIG. 8. This photosensitivity map shows the value of the output voltage (output from the current-voltage conversion amplifier connected to each photodiode) when a beam is scanned along the line X-X' in FIG. 6. CT represents a voltage by a light-receiving current generated by a beam incident on an adjacent photodiode. It is found that crosstalk occurs between photodiodes.
Servo control of the optical disk apparatus is performed on the basis of difference signals from a plurality of photodiodes, e.g., two difference voltage signals Voa+Voc-Vob-Vod and Voe-Vof in FIG. 7. When crosstalk occurs between photodiodes, an error occurs in a disk alignment servo signal, and normal focus servo control and normal tracking servo control fail.
Japanese Patent Laid-Open No. 2-271667 discloses a circuit built-in light-receiving element like the one-shown in FIG. 11. In fabricating this circuit built-in light-receiving element, an n.sup.+ -type buried diffusion layer 32 is formed in a prospective photodiode region on a p-type substrate 31, and a p-type buried diffusion layer 33 is formed in a prospective npn transistor region on the p-type substrate 31. An n.sup.- -type epitaxial layer 34 is grown thick in the npn transistor region on the p-type substrate 31. An n.sup.+ -type buried diffusion layer 35 is formed in part of the p-type buried diffusion layer 33.
An n-type epitaxial layer 36 is grown on the entire surfaces of the photodiode and npn transistor regions. A p.sup.+ -type diffusion layer 37 is formed at the boundaries between respective elements to reach the p-type buried diffusion layer 33. A p.sup.+ -type diffusion layer 37a is formed in part of the n.sup.- -type epitaxial layer 34. In the photodiode region, n.sup.+ -type diffusion layers 38a for a cathode are formed in the n-type epitaxial layer 36 and the n.sup.- -type epitaxial layer 34 to reach the n.sup.+ -type buried diffusion layer 32. In the npn transistor region, an n.sup.+ -type diffusion layer 38b for a collector is formed deep in the n-type epitaxial layer 36 to reach the n.sup.+ -type buried diffusion layer 35, and a p.sup.+ -type diffusion layer 39 for a base is formed in part of the n-type epitaxial layer 36. An n.sup.+ -type diffusion layer 40 for an emitter is formed in part of the p.sup.+ -type diffusion layer 39.
In the circuit built-in light-receiving element having this arrangement, since the n.sup.- -type epitaxial layer 34 is formed thick at a low concentration, the depletion layer can be widened to be longer than the light absorption length, and the adverse influence of the diffusion current can be reduced.
As described above, in the conventional circuit built-in light-receiving element shown in FIG. 8, uniform frequency characteristics cannot be obtained due to the influence of the diffusion current even in the region wherein the frequency is equal to or lower than the cutoff frequency of the circuit. In response to a beam incident on an adjacent light-receiving element, the light-receiving current flows to cause crosstalk between light-receiving elements. To decrease the diffusion current in this circuit built-in light-receiving element, it can be considered that the concentration above the p.sup.- -type substrate 21 is set low to facilitate widening of the depletion layer, or that a general circuit process is also employed. However, the general circuit process cannot be actually employed because the whole circuit process must be improved in order to change the substrate concentration and the thickness of the epitaxial layer.
In the conventional circuit built-in light-receiving element shown in FIG. 11, the diffusion current can be decreased. However, since the n.sup.- -type high-resistivity epitaxial layer 34 must be newly added to the conventional circuit process, the whole process must be improved, and the steps become complicated.